This invention relates to a method and apparatus for remotely detecting surface features and anomalies on wind turbine blades in situ, in particular utilizing digital photographic imaging technology. The invention facilitates such inspections from the ground, on stationary operating wind turbine generators and has utility for remotely detecting propagating latent defects and existing damage on the surface of a wind turbine blade. This permits surface defects to be detected before they become too large for repair in situ, which provides significant economic advantages, as the cost of repairing the wind turbine blade in situ is typically 10% of the cost of replacing a blade.
Due to their large size, extensive surface area and complex shape, wind turbine blades are difficult to nondestructively inspect even within a fabrication or repair facility. Visual inspection cannot identify defects below the surface of the outer skin of the wind turbine blade, which typically is fabricated from a fiberglass material. Active thermography inspection techniques using heat are effective for near surface defects but can give false positives and false negatives due to variations in material thickness and surface emissivity.
Shearography with either thermal stress or during flexure testing of the blade in the factory can be used to detect fiberwaves in spar caps and other areas of the blade, but the technique is slow, expensive and is usually performed only if known issues are suspected. Angle beam ultrasonic techniques are very slow and may not work through thick carbon fiber spar caps.
As a result, blades are commonly installed on towers and put into service with a significant probability of latent manufacturing defects. Furthermore, composite blades are subject to extreme temperature variations. Entrapped water in blades can undergo freeze/thaw cycles, which can cause internal damage. Cyclic forces of gravity and varying forces from the wind acting on the blades as they rotate can cause fatigue damage or the propagation of latent defects over time while manufacturing process mistakes can lead to early blade failure. Defects can grow below the surface of a wind turbine blade to the point that by the time cracks and damage breach the surface and can be detected visually, the damage may not be repairable on tower.
Detecting progressive damage and propagating defects in wind turbine blades in situ is difficult for a number of different reasons. Inspectors using sky cranes or rope access are expensive, time consuming and put personnel in a very dangerous working environment. While on tower, close access allows inspectors to visually detect blade defects such as gel coat cracking, lightning damage, and blade erosion, which may be precursors to further degradation of the blade over time. Access to a wind turbine blade in situ with portable instruments for nondestructive testing also requires rope access or sky platforms and cranes. Blade and tower crawlers with nondestructive testing sensors for in situ inspection have been developed and tested, but they can be extremely expensive, slow to operate.
It is also common practice to use optical and digital photographic imaging of blades in an attempt to detect visible damage from the ground. Standard telephotography often has very low contrast, even with post image processing with current contrast enhancement software. Pixels may be saturated with glare or areas of the blade may be in shadows. The normalization of the histogram, a frequent image processing function, may not be effective in achieving the effect of uniform blade illumination, except in small areas of the blade in the images.
There accordingly exists a need for a fast, cost-effective inspection method and apparatus for wind turbine blades using digital imaging to detect latent and propagating damage, which includes a method and apparatus for precision measurement of features or anomalies and locating these on the target blade.